1. Field of the Invention
The present invention relates to an apparatus for and a method of exposure using electron, ion and other charged particle beams. More specifically it relates to an apparatus for and a method of exposure whereby a sample is exposed while being moved continuously on a stage.
2. Background of the Related Art
Charged particle beam exposure using electron and other beams (hereinafter referred to simply as `electron beam exposure`) is employed when creating fine patterns on LSI chips and similar products. The method requires that the wafer or other material be irradiated with a vast number of fine patterns, therefore its through-put is inferior to that of the conventional stepper. Fine patterns are created in a desired position on the material by deflecting the electron beam to generate the desired pattern, and then deflecting it again to the desired position on the wafer. The problem is that there are restrictions on the range of deflection of electron beams, and when this range is exceeded it becomes necessary to move the stage on which the material, i.e. wafer, is mounted.
However, a method of step and repeat in which the stage is moved and then stopped in order to attain the desired range of deflection necessitates a settling time after the stage has moved and before it comes to a standstill. This settling time is prolonged because the stage is moved mechanically. Consequently, to take an example where the material is a semiconductor wafer and there are 100 deflection areas within one chip, 100 times the abovementioned settling time is required in order to expose before completing the exposure at the point a single chip. This means that the exposure time for one wafer is immense.
For this reason, a method has been proposed whereby instead of being repeatedly moved and stopped in the abovementioned manner, the stage is subjected to continuous movement while the material is irradiated with an electron beam. This is known as the stage-scanning method. The stage is moved at a constant scanning rate during exposure. The result is that if the rate of movement of the stage is set too high, the exposure position at some point ceases to fall out of the area which is capable of being exposed before completing the exposure at the point, with consequent incomplete exposure. Conversely, if the rate of movement of the stage is set low enough to avoid incomplete exposure even in sections where high shot density within the exposure pattern necessitates maximum exposure time, the result is an overall lengthening of exposure and processing time.
This problem can be solved by adopting a method of exposure with variable-rate scanning. In other words, the rate of movement of the stage is varied while the sample is scanned.
Nevertheless, there are numerous problems which remain to be solved in relation to this variable-rate scanning exposure method. Suppose overall exposure time is shortened by setting the rate of movement of the stage as high as is feasible without running the risk of incomplete exposure. If while the stage is moving at high speed it suddenly comes to an area which requires longer exposure, it is not possible to decelerate in time, and this results in incomplete exposure. Controlling the rate of movement of the stage is a problem to which there is not necessarily any simple solution.
As an example of a means of solving this problem, the applicant of the present invention have proposed speed control by feedback as is described in Japanese laid open Patent Tokkaihei 7[1995]-272995. This method takes the optimum rate within the area to be scanned as the initial value, monitors continuously during exposure to check whether it is too fast or too slow, and varies the rate of movement in accordance with the feedback. However, this method fails to provide a necessary control for sudden deceleration as was mentioned above, and it is assumed that such circumstances would result in incomplete exposure.
Another solution which has been proposed calculates the time required for exposure from exposure pattern data and determines the speed distribution of the stage. However, higher levels of integration of LSI mean that the amount of pattern data as such is enormous, and any attempt to calculate exposure time directly from pattern data is flawed because the length of time required for calculation exceeds exposure time. Moreover, beam current values, deflection areas and other properties vary from one exposure apparatus to another, and are subject to variation with time even though the exposure apparatus may be the same, which means that it is frequently not possible to make repeat use of previously calculated speed distributions.
It is an object of the present invention to provide a method of variable-rate scanning exposure and an apparatus for that purpose, so as to solve the abovementioned problem.
It is a further object of the present invention to provide a charged particle beam exposure apparatus and a method of exposure which facilitate the exposure of a material in the minimum exposure time without the risk of incomplete exposure.